Sunshine puts asteroids in a spin

A subtle force from sunlight that changes the spin rate of space rocks has been measured directly for the first time. The feat will help astronomers fine-tune long-term predictions of asteroid orbits and could also explain why some asteroids have a little moonlet in tow.

Sunlight can make asteroids spin so fast they break up, spontaneously shedding a moonlet, concludes Stephen Lowry, a planetary astronomer from Queen's University in Belfast, UK. "They can do it on their own, essentially, which is really fascinating," he says.

Sunlight exerts a gentle nudge on asteroids as they absorb the light on their day sides and radiate it as they rotate, generating a minuscule recoil effect. Essentially, it is like having lots of mini-propulsion engines on the asteroid surface.

Because asteroids are usually irregular in shape, the mini-engines should push one part of the asteroid more than another, creating a twisting force or torque - the so-called YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack) effect.

No one had measured the YORP effect directly. But now two teams have seen the force at work on two asteroids that periodically cross the Earth's orbit.

Spin up

Using various observatories, Lowry and his colleagues monitored the spin rate of a near-Earth asteroid called 2000 PH5 from 2001 to 2005. The rock spins once every 12 minutes or so and is just 120 metres long. "It's like a very odd-looking potato," says Lowry.

The team found that PH5 spun faster over time, knocking 1 millisecond off its rotation period every year during the study, in line with theoretical YORP predictions they tailor-made for this rock.

Watch a movie showing PH5 passing in front of background stars and galaxies over a period of 2 hours in September 2004 (bottom part of frame). The brightness of the asteroid changes as it rotates (courtesy A Fitzsimmons/Very Large Telescope).

Altered orbits

A second team led by mathematician Mikko Kaasalainen from the University of Helsinki in Finland looked at a much bigger Earth-crossing asteroid, 1862 Apollo. This rock is 1.4 kilometres wide and spins every three hours.

Using archived observations recorded between 1980 and 2005, Kaasalainen's team found that 1862 Apollo's spin also quickened, shortening its rotation period by about 4 milliseconds each year, again in line with predictions.

The effect is tiny. It would take about 2.6 million years for YORP to double the rotation rate of 1862 Apollo, for instance.

But because YORP and related effects gradually alter the orbits of asteroids over time, Kaasalainen says it is important for fine-tuning predictions of asteroid trajectories, especially those that threaten to wallop the Earth (see Sunshine sends asteroid off course).

Fly apart

"If you see an asteroid coming fairly close to the Earth and you want to predict exactly how its orbit evolves over a few thousand years or so, these effects would be important," Kaasalainen told New Scientist. "Tiny changes in an orbit can even make a significant difference in 30 years or so."

Lowry adds that the results are important for understanding binary asteroids, space rocks with little moonlets circling around them. Scientists used to think they formed in the wreckage of asteroid collisions or because one rock's gravity captured another.

But a single asteroid could split into a binary all by itself thanks to the YORP effect spinning it up. Lowry predicts that in about 14 million years, PH5 will spin once every twenty seconds or so.

"That's faster than anything we've ever seen," says Lowry. "Depending on how the asteroid is made up - whether it's a solid rock or a rubble pile - there's a really good chance it could fly apart."

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Astronomers have reconstructed the shape of asteroid 1862 Apollo based on measurements of its brightness. Its irregular shape means the sunlight it absorbs and reradiates from different parts of its surface changes its spin rate (Image: Mikko Kaasalainen/Josef Durech)